keratectasia after laser in situ keratomileusis (...
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KeratectasiaAfter
Laser in Situ Keratomileusis( LASIK)
Farid Karimian M.DAssociate Professor of Ophthalmology
Labbafinejad Medical CenterNovember 2006
Case PresentationK.F 20 Y/O student underwent bilateral LASIK for -2.00 D myopia in his both eyes,Pachy OU ∼ 520 µOD had incomplete flap and incomplete operation (No laser!) for 2 times. OS uncomplicated operation
After 10 months:
RFN OD -4.25-1.50x 45° 20/30 Thickness OD 410 µOS plano 20/20
Background
Barraquer (1980) reported Keratectasiafollowing Myopic KeratomileusisConclusion: the Residual stromalthickness is important to prevent Ectasia
Seiler (1998) reported first typical form of Post-LASIK Ectasia
(
Background
Keratectasia is also reported after:* Myopic Keratomileusis* Hyperopic ALK (62%):“Controlled Ectasia* Radial keratotomy * Hexagonal Keratotomy- LASIK , and…PRK (1)
1) Holland SP, Srivannaboons, Reinstein DZ: Ophthalmology 2000:18:177-184
Background
- Progressive Post- LASIK Keratectasia (PPLK) - Iatrogenic Keratectasia- Keratectasia after LASIK
- Immediate (wks) to months afterLASIK generally within 2 yrs
-Peak: 6-10 months
- 0.66%, relative uncommon - Incidence is more than reported
Synonyms:
Onset:
Incidence:
Presentation & Clinical Exam
Positive History; Corneal Refractive SurgeryCorneal Refractive Surgery
Corneal Findings: Anterior & Posterior Corneal SteepeningIrregular astigmatismThinning in the area of Ectasia
General:* Increasing myopia, Progressive
Keratometric steepening* Often with loss of UCVA & BCVAIron ring may be visible Ectasia may appear in one eye while similar treatments in both
Presentation & Clinical Exam
Topographic findings:Central Ectasia
with irregular astigmatismbut good corrected acuity
Paracentral Ectasiaresembling Keratoconus, with irregular astigmatism and poor spectacle-corrected acuity
PathophysiologicalAspects of
Post-LASIK Keratectasia
Regional Anatomical Differences in Normal Cornea
Anterior 100-120 µ of the Corneal Stroma:* More highly compact than posterior* May be more resistant to Mechanical
Deformation Hydration of Cornea and Refractive Index:Changes across Corneal Stromal Thickness
Posterior cornea is unable to withstand normal IOP
1- PPLK as Chronic Disease
Collagenase and Gelatinase activity Increased IL-1 and PGN activity Rearrangement and altered adhesion of lamellae
These are NOT proven for PPLK but:
Loss of Keratocytes in anterior flap and interface Anterior flap metabolic alterations due to Neurotrophic Keratectomy
Is PPLK Similar to KCN??Is PPLK Similar to KCN??
Pathophysiology…
Pathophysiology … cont
A- Early effects of LASIKForward movement of posterior corneal surface(usual finding after LASIK)
Central as well as peripheral cornea is affected after central ablation (C.Roberts Theory)
Posterior stroma after LASIK: * Altered Proteoglycan composition * Fewer Collagen cross links * Reduced Keratocyte Density
(Altered Stromal Remodeling and late Keratectasia)
2- Biomechanical Process
2- Biomechanical Process
IOP is responsible for posterior lamella bulging:Not Proven for normal range IOP (at least in short term) The smaller the radius of curvature the bulging will be smaller
2-Biomechanical Process…cont
B- Corneal Strain Redistribution after LASIK
Cornea is weakest in bend and shear but strongest in tension due to Collagen fibers cross-linking (“stress stiffening”)
Cornea is under strain even in normal physiological conditions
After ablation:* Posterior lamellar tension is increased* Reduced interlamellar cohesive strength in the
Infero-Central Cornea (where Ectasia is common)
C - Corneal Shape Changes after LASIK
Peripheral cornea moves outward and forward (Roberts’ Rubber-Band Theory)
Central Corneal MovementLimbus doesn’t expand Surface area of posterior cornea remains constant
Transverse contraction of posterior lamellae(Poisson’s ratio)
(V= transverse strain / longitudinal strain)
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D - Corneal Resistance to Fatigue
Cornea has Continuous Resistance to:IOP
Deforming Forces of EyelidsKeep stable focusing of Light on Retina
Repeated Microtrauma e.g. Eye Rubbing
Dynamic Fatigue Ectasia
3- Combined process of Fatigue and Proteolysis
Static & Dynamic fatigue process of Cornea
Mechanical failure
Enzymatic Proteolysis Subclinicalinflammation
Molecular Collagen changes Adhesion Protein changes
PPLK
Risk factors for PPLK in LASIK
Depends on other parameters Large Residual stromal bed
Depends on other parameters Low Attempted correction
>6mm Small Ablation diameter
>160 µThin Flap thickness
<500µThick Preop. Corneal thickness With irregular topography Small Curvature radius
(1-v)P/E > 0.001Low Loading pressure /elasticparameters
>20mmHgLow IOP(1-v)/E>0.7MPa-1Low Poisson Ratio <1MPaHigh Young modulus
Warning SafeParameter
Theories about the Risk Factors
for Keratectasia
1- Pre-existing Keratoconus or Forme Fruste Keratoconus
FFKC: - is abortive or early form of KCN - Stable refraction and Corneal
Curvature - 30% of Keratectasia cases had preop
Forme Fruste KCN (FFKC)
Post-LASIK Ectasia topography is different:Preop Normal eyes Central steepeningPreop FFKCN eyes Inferior steepening
2- Minimum Residual Stromal Thickness
Low Residual stromal thickness is mechanically Unable to withstand the Intraocular Pressure
Subtraction Technique:Is Not Reliable
Remained stromal thickness (RST)=Corneal thickness (Pachymetry) – (flap thickness + ablated depth)
Flap thickness:- Inaccurate with different microkeratomes- Thick flap doesn’t prevent ectasia even
predispose to EctasiaAblated thickness (depth): variable with Hydration
Anterior flap does not contribute to the biomechanical stability of cornea
Direct RST Measurement
Is NOT accurate Laser ablation dehydrates stroma Overestimation of actual tissue removal Under- estimation of RST Anterior bowing of the posterior cornea occurs after LASIK For Enhancement: RST must be determined after flap removal because of:
* Inaccurate flap thickness* Epithelial hyperplasia
Minimum RST to prevent Keratectasia
Variable Thickness reported: 200-320µ>18% corneal thickness ablation: increases risk
Postop Corneal Thickness must be 55-60% preopthickness but not less than 475- 500 µ
RST safety index: * > 300 microns Safe zone* 250-300 microns Borderline zone* < 250 microns “Danger zone”
3 - Enhancement Procedures
Cornea tissue ablation will decrease RST
%22 percent of Ectasia cases had at least one enhancement procedure
Regression! of myopia with or without astigmatism may be initiation of Ectasia
If enhancement indicated, up to 2-3 diopterscan be enhanced by ablation over the flap
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4- Preoperative Corneal Thickness
Current standard for pachymetry: Ultrasound PachymetryOrbscan II (Optical slit):
- Mostly overestimates 20-30 µ- Only one report shows underestimation - Calibration Coefficient Factor: 0.92-0.96
- More unreliable in “non-virgin” Corneas
Orbscan is Not suitable for Reoperation
Thin corneas with higher IOP may have a risk for forward shift of Posterior Corneal Surface after LASIK
Posterior corneal surface shift is larger inthose with lower preop Corneal Thickness, higher IOP and greater Myopia
4 - Preoperative Corneal Thickness
5- Thick Corneal Flap
Same Microkeratome and same blade produce variable flap thickness
Preoperative thin corneathinner flap
Thick flap high risk forimmediate Corneal Ectasia(even without ablation!!)
6- Laser Corneal Tissue Resection
Corneal tissue removed by the excimer laser Inaccurate immediate post-Laser intraoperative pachymetryLaser resection is variable with laser type and algorithms Munnerlyn’s formula: resected thickness depends on diameter, ablation profile (optical zone) and diopters
Most of Keratectasia have been in Myopia over – 6.0D correction
Exception: Some eyes with low attempted Correction have developed Ectasia
Correction of regular Astigmatism does not increase the risk for Ectasia
7- Attempted Refractive Correction
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Faraj and coworkers:“ An ablation diameter of 6.0 mm or greater is a risk factor”
Difficult to conclude; “diameter alone is a risk factor!!”The larger the diameter of ablation, the more tissue is removed
Increasing the ablation diameter increases thedeformation of the posterior corneal surface
8- Diameter of the Ablation Zone
History of Contact Lens useRGP contact lens wearers before LASIK need more Enhancement surgery Most of Keratectasia cases had no history of contact lens wearing: no direct relationship
Chronic Eye RubbingMay be a risk factor for development of ectasia
9- Other Possible Risk Factors
Prevention
Prevention is simpler than Treatment
1- Corneal Topography & Orbscan
- Is Mandatory preop to detect FFKN and KCN - Standardized map is more accurate than absolute scale topography Always Consider Corneal Warpage as unstable and unreliable evaluation
Prevention
KCN diagnosis:Central Pachymetry Difference: >20 microns Decrease in midperipheral Corneal ThicknessAbnormal Posterior Corneal Elevation(>40 microns) in Orbscan
2- Family historyPresence of history of KCN or FFKN in family: “ May be an Alarming sign” !
Intraoperative Preventive Measures
Measurement of posterior stroma (before and after ablation)
Reduction of flap thickness (Thin-flap LASIK)Change procedure to surface ablation
(in borderline cases)Reducing optical zones as far as possiblePlanning enhancement procedures with suspicions and caution
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Postoperative Measures for Prevention
Advise the patient not to rub the eyes even in long term Myopic regression may be due to Ectasia
Also consider myopic regression may be due to lens Nuclear Sclerotic (N.S) changesPre-ectasia condition must be diagnosed by topography or OrbscanTopical IOP lowering medications: temporary!!
Depends on the Severity and Typeof Keratectasia:Glasses* May be helpful in early Stages of Central type* Patient Not complete satisfied * May be a transient management
Treatment
Soft Contact Lens:- may temporarily improve vision - Can be used in those intolerated RGP
RGP Contact Lens:- Ideal due to correction of astigmatic irregularity - More difficult fitting than Keratoconus patients - Usually patient not satisfied
(previous CL-intolerant, psychological)
Treatment
Contact lenses
Excimer Laser Customized Treatment- Not effective, not proven - Unpredictable, poor outcome
Intracorneal segments (ICS or ICR)- Reduction of astigmatic irregularity and myopia- Short term effective - Difficulty in procedure (in comparison to KCN)
Treatment
Deep Anterior Lamellar Keratoplasty (DALK)- New modality- Needs more Surgical experience - Prolonged Visual Rehabilitation
Penetrating Keratoplasty (PKP)- May need future Refractive Surgeries - Will have its specific problems
Treatment
Keratoplasty
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Conclusion
Post-LASIK Keratectasia is a SeriousComplication of LASIK
There are Multiple Risk Factors which Most of them can be Screened and Diagnosed before and Intraoperatively
Management of this condition is Difficult,Try to Prevent its Occurrence !!
THANK YOU FOR YOUR
KIND ATTENTION!!
Questions…?